1. Field of the Invention
The present invention relates to a shift control system of an automatic transmission adapted to appropriately alter a shift characteristic of the automatic transmission.
2. Description of Related Art
Recently commercially available automatic transmissions are generally designed so as to have its shift characteristics altered in an appropriate way. For such automatic transmissions, there are prepared two previously mapped shift characteristics, for example, an economy mode which is mileage-oriented and a power mode which is output-oriented and in which the shift line is set on the side of a high vehicle speed rather than the economy mode, thereby performing a shift control under a desired shift characteristic manually selected.
Japanese Patent Unexamined Publication (kokai) No. 8,983/1982 discloses a shift control system in which the shift characteristic is automatically altered in accordance with the load of the engine. Specifically, the shift control system performs a shift control on the basis of a pre-mapped economy mode at an ordinary driving state, while the shift control system alters the shift control to the output-oriented shift characteristic on the basis of the shift characteristic of the economy mode when the load of the engine is equal to or larger than a predetermined reference value, for example, when a throttle valve is three quarters or more open. The alteration of the shift characteristic forms the shift characteristic for the power mode by altering the shift line for the economy mode to the side of a high vehicle speed in a control circuit on the basis of the shift characteristic of the economy mode. In other words, the load of the engine above the reference value means the requirement for a large output (torque) of the engine, and the output-oriented shift characteristic is automatically set. Further, this conventional technique is such that the shift characteristic is returned from the power mode to the economy mode when the vehicle speed becomes smaller than a predetermined value or a foot brake pedal is depressed.
However, in this conventional technique, the reference value to be used when the shift characteristic is altered is set to a certain constant value, so that the shift characteristic cannot be altered to an appropriate one so as to satisfy the requirement for acceleration by the operator.
This matter will be described more specifically. For instance, when the vehicle is running at a stationary speed with the load of the engine on the orderof a value somewhat exceeding the reference value, the shift characteristic is altered to an output-oriented power mode although the economy mode is preferred from a standpoint of mileage or the like. To the contrary, when the current vehicle speed at which the vehicle is running at a stationary speed is considerably smaller than the load of the engine corresponding to the reference value, the shift characteristic is not altered and it is in the economy mode as it is as long as the load of the engine does not reach the reference value or higher, even if an accelerator pedal is depressed, and no sufficient degree of acceleration can be obtained.
Japanese Patent Unexamined Publication (kokai) No. 22,698/1976 discloses another conventional technique in which the shift line is altered immediately when the accelerator pedal is depressed rapidly.
However, for this conventional technique in which the shift characteristic is altered immediately in accordance with a change rate of the load of the engine, the output-oriented shift characteristic is automatically set because the change rate of the load of the engine becomes too large even if the load of the engine is in an absolutely small range, namely, even if no output (torque) of the engine is required to that large extent. To the contrary, even in a range in which the load of the engine is absolutely large, namely, even when the output (torque) of the engine is required, the mileage-oriented shift characteristic is automatically set due to a small change rate of the load of the engine.